Enter the text here that is the new abstract information for your application. This section must be no longer than 30 lines of text. Cytoplasmic dynein is a microtubule motor protein involved in a very wide array of cellular functions, including vesicular transport, chromosome segregation, and cell migration. A single major form of cytoplasmic dynein is responsible for almost all aspects of these activities, but how it is adapted to such a diversity of functions at a broad range of subcellular sites remains a major question. Among the known dynein interactors, two complexes have emerged with prominent roles in dynein cargo binding and motor regulation, NudE-LIS1 and dynactin. We have recently reported NudE-LIS1 to have a novel and unique ability to interact with the dynein motor domain during its powerstroke and adapt the motor protein for high load functions. We also found LIS1-NudE to compete with dynactin for dynein binding, suggesting that the two complexes may serve in alternative regulatory roles. This proposal is to test this hypothesis and to bring to bear on dynactin the approaches we have successfully used to determine the functions and mechanism of action of LIS1 and NudE. Dynactin is known to enhance dynein processivity in vitro. However, its interaction with dynein has been difficult to control, hampering progress toward a complete understanding of its mechanochemical functions. Because dynactin is also important in dynein cargo recruitment, its specific in vivo role in motor regulation has also been difficult to define. Preliminary results have identified conditions controlling the dynein-dynactin interaction, and have revealed potent, long-range allosteric effects for dynactin fragments on dynein force production and processivity. The Aims of this proposal are (1) to determine how the dynein-dynactin interaction is regulated and to produce and define cocomplexes for further analysis; (2) to determine the complete scope of regulatory functions for the dynactin complex, its major regulatory subunit p150Glued, and its subfragments to understand the underlying mechanisms for dynein regulation; and (3) to determine the specific roles of dynactin in dynein motor regulation in vivo by high resolution particle tracking and force analysis. These studies are of broad relevance for understanding basic mechanisms of cell behavior. In addition, they should shed important new light into the mechanisms underlying brain developmental disease, motor neuron degeneration, cell division, and other physiological and pathophysiological functions.